1. Field of the Invention
The present invention relates to an apparatus and method for supporting distortionless vector perturbation in a Multiple-Input Multiple-Output (MIMO) system. More particularly, the present invention relates to an apparatus and method in which a symbol having a maximum Signal to Noise Ratio (SNR) is transmitted by generating a plurality of labels and candidate sequences for one input signal and then generating independent symbols by adding a limited offset vector.
2. Description of the Related Art
Recently, the use of a Multiple-Input Multiple-Output (MIMO) technique in a mobile communication system has expanded, particularly because the MIMO technique supports a high data rate. In addition to a point-to-point MIMO technique, there is active research on a multiple-user MIMO technique in which a Base Station (BS) transmits data to a plurality of Mobile Stations (MSs) corresponding to multiple users. Unlike the point-to-point MIMO technique, in a multi-user environment, data is transmitted simultaneously to the multiple users in order to increase transmission efficiency. In such a case, when the number of Transmit (Tx) antennas is greater than the number of Receive (Rx) antennas, a multiplexing efficiency can be maximized only when data is simultaneously transmitted to the multiple users. One problem which may occur in this case is that a signal of one user may act as interference to another user.
A method of canceling an interference signal may be classified into an interference cancellation method of a transmitting end and an interference cancellation method of a receiving end according to the location where the interference signal is cancelled. When the number of Tx antennas is one, interference is canceled at the receiving end. That is, a method in which a signal of another user is decoded and is then cancelled in the receiving end is provided as an optimal method without having to use an interference cancellation method of the transmitting end. On the other hand, when the number of antennas of the transmitting end is greater than one, it is not optimal to cancel interference in the receiving end. Thus, an interference cancellation method of the transmitting end is performed. If interference cancellation is performed at the transmitting end, Dirty Paper Coding (DPC), which supports a maximum data rate, has been shown to provide good performance. An example of DPC is a vector perturbation method. The vector perturbation method is a pre-coding technique for a broadcasting channel of a Gaussian MIMO system and has recently been emerged as an important research topic. The vector perturbation method is considered a promising technique because it has a desirable computational complexity and provides almost all the capacity of the broadcasting channel.
In the vector perturbation method, a Signal-to-Interference plus Noise Ratio (SINR) is maximized by using an integer offset vector. An optimal offset vector is obtained by solving an integer programming problem which restricts all variables to have only integer values. Because an Rx signal is affected by vector perturbation, the vector perturbation is removed by a receiver which performs a modulo operation and thus prevents distortion from being added to the signal.
There is another vector perturbation method in which an integer offset vector and a continuous offset vector are used together to reduce a total Mean Squared Error (MSE). The introduction of the continuous offset vector results in a decrease in the MSE. However, unlike the integer offset vector, the continuous offset vector causes distortion in the receiver. Accordingly, the offset vector may be optimized by balancing between the decrease in the MSE and an amount of distortion. The optimal offset vector is determined in two steps. First, the integer offset vector is obtained by solving the integer programming problem. Second, the continuous offset vector is obtained to have a format similar to the integer offset vector obtained by solving the integer programming problem. However, in either the case of using only the integer offset vector or the case of using both the integer offset vector and the continuous offset vector, the integer programming problem must be solved, which leads to a computational complexity.
Accordingly, there is a need for an apparatus and method for supporting vector perturbation that is capable of providing improved performance, as compared to a conventional method of the vector perturbation, by reducing a computational complexity while increasing an SINR.